Gov. Deval L. Patrick has turned to William “Mo” Cowan, his former chief of staff, friend and close adviser, to serve as the interim U.S. senator to fill the vacancy created by U.S. Sen. John F. Kerry's resignation to become secretary of state.

“Today I have the great honor, privilege and personal pleasure to appoint Mo Cowan as United States senator,” the governor said, introducing Mr. Cowan at a Statehouse press conference that ended weeks of speculation over who would be chosen as an interim replacement for Mr. Kerry.

Mr. Cowan, who lives in Stoughton with his wife, Stacy, and two young sons, will serve until voters elect a replacement June 25. That senator's term will run until January 2015, the end of Mr. Kerry's term.

A primary, if needed, would be held April 30.

Mr. Cowan, whose nickname is derived from his middle name Maurice, becomes only the second African-American to represent Massachusetts in the U.S. Senate. Mr. Cowan accepted the appointment saying he was honored and humbled, while pledging to work “as hard as humanly possible” to represent the people of the state and asserting that he has no plans to ever run for office.

Mr. Kerry's nomination as secretary of state was confirmed by the U.S. Senate Tuesday, and his resignation is effective Friday.

The governor's selection of Mr. Cowan as the interim senator fills in one more blank in the unfolding process to replace Mr. Kerry in the Senate.

Speculation about a replacement had included former U.S. Rep. Barney Frank, who retired from the House this month and indicated he was interested in the interim post. Many Republicans expect former U.S. Sen. Scott P. Brown to make a decision in the coming days on whether to try win back a seat in the Senate after losing a bid for a full term to Democrat Elizabeth Warren in November.

While U.S. Rep. Edward Markey, D-Malden, has announced his candidacy in the special election and been endorsed by Mr. Kerry, U.S. Rep. Stephen F. Lynch, D-Boston, has made plans to announce he will also run for the Democratic nomination at campaign-style stops Thursday in Springfield, Worcester and Boston.

Mr. Lynch's Worcester stop will be at 11:30 a.m. at the Parkway Diner, 148 Shrewsbury St.

Mr. Cowan said he knows the people of the state are interested in jobs, education and affordable and accessible health care and said his efforts will be “closely aligned” with the governor's.

He said he expects people will likely see “no daylight” between his positions and Mr. Kerry's.

Mr. Cowan, 43, is a Boston lawyer and worked as the governor's chief legal counsel briefly in 2009 before becoming his chief of staff in January 2010. Having worked at the highest levels of state government on key issues and engaged with a wide range of interest groups across the state, he said, he has developed a deep understanding of key issues facing Massachusetts.

In terms of issues facing the Senate, he said he will support “a balanced approach” to the deficit-cutting decisions ahead in the coming months specifying that he will prefer a mix of spending cuts and revenue increases over the automatic across the board spending cuts that will take affect this spring without new budget action by Congress.

Mr. Cowan was among a number of the governor's cabinet members who chose not to continue in that capacity office for the final two years of the governor's term. He left the post at the end of last year, but remained an adviser to Mr. Patrick.

Mr. Patrick said Mr. Cowan served “on the front lines in our efforts to manage through the worst economy in 80 years” and earned the respect and admiration of people throughout government while working in the governor's office.

“The people of the commonwealth have benefited from his wisdom and good judgment during his time in our office, and will again in the Senate,” Mr. Patrick said.

Mr. Cowan, who grew up in North Carolina and graduated from Duke University, recalled being raised by his mother after his father died when he was a teenager. He described her as a single mother and “a child of the segregated South,” who never got the chance to go to college, but taught him hard work would create opportunity.

“My mother told me days like today were possible. If you work hard and treat people with respect, there is very little you cannot achieve in this great nation,” Mr. Cowan said. He noted that his mother is recovering from knee surgery in North Carolina and that he hoped she got to see a video recording of the announcement.

He said he came to Massachusetts after graduating from Duke to attend Northeastern University School of Law.

Every day since he said, “I have had opportunity to grow personally and professionally. I met my wife here. I have my family and built a home here. Massachusetts is the commonwealth of opportunity,” he said.

Lt. Gov. Timothy P. Murray expressed his confidence in Mr. Cowan and commented on the sharp and stylish persona of Mr. Cowan, who often wears bow ties with his suits.

“Anyone who has had a chance to work with Mo Cowan knows you can't find a better individual — smart strategic, tough, just all the things that will lend themselves well in representing the commonwealth,” Mr. Murray said. “Lastly he's cool. Tom Brady, George Clooney, James Bond, the president have nothing on Mo,” Mr. Murray said.